This invention relates to television AFC systems. The television broadcast signal comprises a carrier upon which luminance, chrominance and sound information is modulated within a limited bandwidth. The sound information is present as a frequency modulated carrier displaced from the station carrier. In the NTSC signal transmission used in the United States of America, the sound and picture carriers are frequency-spaced 4.5MHz while the channel bandwidth is 6MHz. In order to fit the frequency-spaced sound and picture carriers and other information within the prescribed bandwidth, vestigial transmission in which one sideband (in this case the lower sideband) is substantially attenuated with respect to the other sideband (upper sideband) is used. The range of broadcast frequencies assigned to television transmission are not continuous but rather forms an interrupted group of bands. However, despite this discontinuity, all assigned channels have at least one adjacent channel, and more typically two adjacent channels "flank" each channel. This means that the majority of channels have adjacent sound carrier signals 6MHz above and below their desired sound carrier and adjacent picture carriers 6MHz above and below their desired picture carrier.
The vast majority, if not all, currently manufactured television receivers include a tuning system which selects a desired channel by frequency converting the received broadcast signal, using the well-known heterodyning process, to a common intermediate frequency (IF) signal having frequency-spaced picture and sound carriers. Unfortunately, the heterodyning process also frequency converts the adjacent channel carriers to "intermediate frequency" signals. As a result, most receivers use frequency selective intermediate frequency filters which not only pass the desired intermediate frequency signal but also include trap networks which exclude or attenuate undesired adjacent channel information.
For example, in the system of assigned frequencies within the U.S. a standard IF frequency of 45.75MHz for the picture carrier has been generally established. Correspondingly, the sound carrier associated with the picture carrier is 4.5MHz lower in frequency at 41.25MHz. In the portions of the television band in which adjacent channels are present, the associated IF sound carrier of the lower adjacent channel is 47.25MHz (only 1.5MHz away from the desired channel picture carrier) while the associated picture carrier for the upper adjacent channel is 39.75MHz (only 1.5MHz away from the desired channel sound carrier). One can readily see that correct frequency conversion which accurately places the selected channel picture and sound carriers at the desired IF frequency and the undesired adjacent picture carriers at the associated trap frequencies is subject to stringent requirements.
These and other stringent tuning requirements have lead practitioners in the television art to utilize automatic frequency control systems (AFC) which maintain the correct frequency conversion within the tuning system. Such AFC systems are well-known in the art and are of almost endless variety but all may be characterized by the performance of two essential functions. The first function is generally called "pull-in" or "frequency acquisition" in which an existing frequency deviation of the picture IF carrier from the desired 45.75MHz is corrected by the closed loop response of the AFC system. The second function is generally called "hold-in" which involves the maintenance of correct frequency conversion (i.e., synchronization) once frequency acquisition has been accomplished. A basic limitation in the ability of most AFC systems to acquire correct tuning in the face of a substantial frequency deviation arises from the presence of the adjacent channel picture and sound carrier traps described above. For example, when the oscillator frequency is displaced such that the frequency conversion results in "placing" the video carrier within the adjacent channel sound trap, virtually no error signal, or control effect, is produced within the AFC system due to the picture carrier. However, the sound carrier under such conditions is "exalted" by the IF filter response and produces substantial energy within the AFC system resulting in an erroneous control voltage.
In most AFC systems, dominance by the sound carrier rather than the picture carrier, causes the system to lose its pull-in capability and "lock-out" of the system occurs. Similarly, the relative signal strength of picture and sound carriers at correct tuning is determined by the IF filter response, and under proper transmission conditions the effect of the picture carrier will dominate the AFC system. However, transmission problems such as multi-path interference or "tilt" within the antenna and distribution system can disturb this relationship resulting in the production of an overriding control effect produced by detection of the sound carrier which again can produce a lock-out condition.
The problem of AFC lock-out through receiver detuning causing intrusion of the picture carrier into the adjacent channel sound trap has been minimized by development of AFC systems in which the sound carrier produces a control effect of the proper polarity to aid or complement that produced by the picture carrier and actually control the AFC system when the frequency deviation places the picture carrier within the adjacent sound trap. One such system shown in U.S. Pat. No. 3,459,887 uses an automatic frequency control system in which the balance of a diode-pair AFC detector is offset, or biased, to produce the desired complementary control effect by the sound carrier. The described system achieves substantial improvement in AFC pull-in when the picture carrier is attenuated by the adjacent channel sound trap. A somewhat similar system is shown in U.S. Pat. No. 3,968,325 in which a product detector, or multiplier, simultaneously driven by a pair of IF signals emmanating from the intermediate frequency filter performs the AFC detection function. A frequency-dependent phase shift between the two IF signals is introduced such that the frequency deviation of the intermediate frequency signal is converted to a phase deviation to which the product detector responds. The AFC response provides a reduction of the erroneous AFC voltage produced by detected noise in the region of the received channel sound carrier and a complementary sound carrier control effect similar to that of the U.S. Pat. No. 3,459,887. The creation of a complementary sound carrier control effect in both systems provide improved pull-in or acquisition when the frequency deviation is such that the picture and sound carriers are above the correct frequencies. However, such systems do not produce complementary control effects when the frequency deviation is low.
Another problem caused by sound carrier detection in all AFC systems whether complementary or not is the production of an offset voltage when the receiver is correctly tuned. Fortunately, in the majority of receivers, the amount of sound carrier energy applied to the AFC system at or near correct tuning is reduced by the presence within the IF response of a sound trap used to inhibit the production of excessive chrominance-sound beat caused by detector non-linearity. As a result, the degree of sound carrier frequency offset is greatly reduced. However, the more recently employed synchronous detection systems have improved detection linearity and very little chrominance-sound beat signal is produced. This is advantageous because it allows the use of an extended bandwidth intermediate frequency filter, that is, a filter without a trap network for the attenuation of the sound carrier. The benefits realized by such extended bandwidth IF systems are numerous. For example, more linear signal translation, particularly the chrominance information, is achieved and a greater range of receiver tuning is tolerated by the system. Unfortunately, the removal of the sound trap from the IF response also allows substantial sound carrier energy to reach the AFC detector and produce a substantial offset voltage which under some circumstances mistunes the receiver.
Accordingly, it is a general object of the present invention to provide an improved automatic frequency control system. It is a more particular object to provide an improved automatic frequency control system in which a complementary control effect due to sound carrier detection is realized under conditions of high and low frequency error. It is another object of the present invention system to provide an improved automatic frequency control system for use with extended bandwidth intermediate frequency filters.